Focal cartilage defects occur due to trauma, age-related degeneration and other causes, and can lead to progressive cartilage degeneration culminating in painful degenerative arthritis. These defects present a critical medical concern due to the limited capacity of cartilage to self-repair, and the limited success of current approaches in robustly repairing cartilage defects over the long term. One promising approach to repair defects is the use of tissue engineered cartilage-like equivalents formed in vitro to functionally replace damaged cartilage. Tissue engineered cartilage-like equivalents are formed by encapsulating cells within 3D hydrogels, and culturing the gels in media that contains the appropriate biochemical cues. While healthy chondrocytes encapsulated within hydrogels can form cartilage-like tissue equivalents, there is limited availability of healthy chondrocytes from patients with osteoarthritis and significant donor site morbidity during harvesting can occur. Mesenchymal stem cells (MSCs) present an attractive alternative cell source for forming tissue engineered cartilage equivalents. MSCs have been found to undergo chondrogenic differentiation when given the appropriate biochemical cues and encapsulated in hydrogels. However, MSC based tissue engineered constructs fail to mimic natural articular cartilage tissue in terms of their composition and mechanical properties. Here we propose to develop hydrogels that are viscoelastic, exhibiting fast stress relaxation, and engineered degradation for MSC-based tissue engineered cartilage. The specific hypothesis to be tested in this proposal is that fast stress relaxation combined with full degradability of hyaluronic acid (HA)-based hydrogels will direct chondrogenic differentiation of MSCs and promote formation of an interconnected cartilage matrix having mechanical properties, composition, and architecture approaching that of natural articular cartilage tissue. The proposed study will build on work by the PI?s group that have demonstrated the development of HA hydrogels with fast stress relaxation, and have shown that fast stress relaxation in alginate hydrogels promotes cartilage matrix formation by chondrocytes. This hypothesis will be tested in two specific aims: (1) materials design: develop hyaluronic acid based hydrogels in which degradation rate, stress relaxation, and stiffness can be independently modulated using a set of modular components; (2) in vitro testing: determine the optimal levels of degradation and stress relaxation for formation of engineered cartilage by human MSCs. This approach is innovative because the development of hyaluronic acid based hydrogels with both engineered degradation and stress relaxation represents an innovative strategy in biomaterials design, and presents a new type of biomaterial for cartilage tissue engineering. The proposed research is significant because of its potential to provide the critical advance for forming tissue engineered cartilage equivalents by MSCs, and in the ability of this approach to be translated into the clinic. A robust approach to repairing cartilage tissue defects will have a tremendous impact on the quality of life for patients, and reducing health care costs long-term.